In commercial preparations, the compound of formula ##STR10## or 3a.alpha.,5a.beta.,9a.alpha.,9b.beta.-dodecahydro-3a,6,6,9a-tetramethyl-na phtho[2,1-b]furan, known under the commercial name of AMBROX.RTM. (origin: Firmenich SA, Geneva), is often accompanied by variable amounts of its diastereoisomers, amongst which epi-AMBROX and iso-AMBROX.
Ever since its discovery [see Helv. Chim. Acta 33, 1251 (1950)], there have been many reported processes for the preparation of this compound. Said processes are generally based on a reaction of oxidative degradation of terpenes such a (-)-sclareol or (+)-manol, or on the use of ambriene as starting material [G. Ohloff in "Fragrance Chemistry", ed. Ernst T. Theimer, p. 545 and following, Academic Press (1982)]. All these materials are of natural origin and therefore their availability and quality are dependent on variable climatic conditions and particular socio-economical factors.
In addition, since they are extracted from natural sources with modest yields, they are available at a price which renders their use on an industrial scale uneconomical.
A process disclosing the cyclization of homofarnesic acid into norambreinolide, followed by the reduction of the obtained lactones and the cyclization of the resulting diol to provide the desired furan derivative, has been reported in European Patent no. 107 857. According to this process, the cyclization of homofarnesic acid is achieved by means of SnCl.sub.4.
A similar synthetic approach had been suggested by A. Saito et al. [Chemistry Letters 757 (1981)]. These authors had realized the cyclization of trans-.beta.-monocyclohomofarnesic acid using same SnCl.sub.4 as cyclization agent. The two cited documents, incidentally, represent an extension of the work done by G. Lucius on the cyclization of homofarnesic acid [Angew. Chem. 68, 247 (1956); Arc. Pharm. 291, 57 (1958) and Chem. Ber. 93, 2663 (1960)].
Recently, S. Neumann and H. Simon [Biol. Chem. Hoppe-Seiler 367 (8), 723 (1986)] described the formation of 3a,6,6,9a-tetramethyl-perhydronaphtho[2,1-b]furan by an enzymatic cyclization of homofarnesol or of homofarnesyl-(1,5,9-trimethyl-4,8-decadienyl)ether. However, for the moment, such a process has only a purely academic interest, due to the particular nature of the reagents employed, as well as to the observed yields and conversion rates.
The special contribution that AMBROX makes to the perfume industry and the need to have it at a lower market price prompted us to re-examine the available processes for its synthesis. The present invention brings a novel and original solution to this problem.